Due to the two-barrier philosophy for subsea hydrocarbon wells, an upper and a lower plug are conventionally installed in the bore of a subsea X-mas tree or its internal elements, such as the tubing hanger. It is known to replace the upper plug with an internal tree cap, the tree cap having features in addition to just blocking for fluid connection.
Known internal tree caps are installed and retrieved through a marine riser from a surface installation. Thus, such installing or retrieving operations are cumbersome since they require the establishment of the riser from the surface and down to the well tree. Establishing the riser takes time and one needs to use a rig. Rigs are not always easily available and are also expensive to rent on a day-to-day basis.
In addition, in some instances the PTV-line (plug testing valve) can be blocked, making it impossible to test the space between the lower plug and the internal tree cap. With a conventional internal tree cap run inside a riser, one faces difficulties solving such problems accompanying such situations. For instance, retrieval of the internal tree cap can possibly not be done due to the hydrostatic lock formed by the sealed-off space below the internal tree cap.
International patent application publication WO2007054644 describes a cap for a subsea tree and for use with a tubing hanger. This cap is adapted to be arranged both internally and externally about the tree spool, and is not adapted to be landed through a marine riser. It is adapted to be landed on a wire.
Furthermore, patent application publication US20040216885 describes a method for installing a tree cap on a subsea Xmas tree with the use of an ROV. The cap has a channel through it in order to provide a vacuum or negative pressure in the space below the cap, thereby “sucking” the cap into place.